(1) Field of the Invention
The present invention relates to a main bearing for a crankshaft of an internal combustion engine, and more particularly relates to a main bearing for supporting a journal portion of a crankshaft in a cylinder block lower portion of an internal combustion engine. Further, the present invention also relates to a bearing device which is configured by such a main bearing and a shaft section corresponding thereto.
(2) Description of Related Art
A crankshaft of an internal combustion engine is supported at a journal portion thereof in a cylinder block lower portion of the internal combustion engine via a main bearing formed by a pair of semi-cylindrical bearings. In order to lubricate the main bearing, lubricating oil pumped by an oil pump is fed through an oil gallery which is formed in a wall of the cylinder block and a through port which is formed in a wall of the main bearing into a lubricating oil groove which is formed along an inner circumferential surface of the main bearing. In the crankshaft, a first lubricating oil path is formed to penetrate the journal portion in a diameter direction, and communicates with the lubricating oil groove of the main bearing though both end openings thereof. Furthermore, a second lubricating oil path is formed to branch from the first lubricating oil path and pass through a crank arm section, and communicates with a third lubricating oil path which is formed to penetrate a crankpin in a diameter direction thereof. Accordingly, the lubricating oil having been fed into the lubricating oil groove of the main bearing passes through the first lubricating oil path, the second lubricating oil path and the third lubricating oil path, and thereafter, is supplied to a slide surface between the crankpin and a connecting rod bearing from an end portion opening (lubricating oil outlet which is formed on an outer circumferential surface of the crankpin) of the third lubricating oil path.
The lubricating oil groove of the main bearing is formed on an inner circumferential surface of at least one of a pair of semi-cylindrical bearings throughout an entire length in a circumferential direction thereof (see FIG. 1 of JP-Y2-61-00573). In this case, the lubricating oil which is supplied to the lubricating oil groove of the main bearing from the oil gallery in the cylinder block mainly flows to the end portion in the circumferential direction of the semi-cylindrical bearing in accordance with rotation of the journal portion, and most of the lubricating oil is discharged to an outside of the bearing through the axial groove formed in the joint portion of the pair of semi-cylindrical bearings.
Furthermore, in recent years, in order to reduce the leakage amount of lubricating oil from an end portion in a circumferential direction of a semi-cylindrical bearing, in response to miniaturization of a lubricating oil supplying oil pump for the purpose of enhancing fuel efficiency of an internal combustion engine, there is used a main bearing in which the length in the circumferential direction of a lubricating oil groove is configured to be shorter than the entire length in the circumferential direction of the semi-cylindrical bearing, and therefore, at least one of both end portions in the circumferential direction of the lubricating oil groove does not extend to the end portion in the circumferential direction of the semi-cylindrical bearing (namely, does not open at an axial groove) (see JP-Y2-61-00573, JP-A-04-219521, JP-A-2005-249024, JP-A-2011-179572 and JP-A-2008-095858).